This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 10-241863 filed on Aug. 27, 1998, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention is related to soldering paste, a soldering method, and a surface-mounted type electronic device.
2. Related Arts
When a conventional flux containing rosin as a base and aminohalogen salt as an activator is used for soldering paste, the base and the activator remain after reflow. This gives rise to problems that (i) an insulation property between electrodes is insufficient when a narrow electrode distance of approximately 100 xcexcm is taken, (ii) corrosion occurs when a mounted component is used in an environment permitting contact with air, and (iii) silicone gel is not hardened when the mounted component is sealed therewith.
To solve these problems, Japanese Patent Application Laid-Open No. 9-94691 proposes a volatile polyhydric alcohol material having two or more OH groups as a base of flux for soldering paste. In this case, when specific alcohol is selected as the polyhydric alcohol, satisfactory wettability can be secured by reducibility of this material without adding an activator. Under these circumstances, there is a requirement of further improving continuous printability.
The present invention has been made in view of the above-mentioned problems. An object of the present invention is to improve printability of soldering paste without lowering wettability and insulation reliability. Another object of the present invention is to provide a soldering method for soldering an electronic component to a base member using soldering paste with high printability and wettability. Still another object of the present invention is to provide a surface-mounted type electronic device including an electronic component surface-mounted on a base member with high insulation reliability.
According to a first aspect of the present invention, a soldering paste is composed of a flux including alcohol as a base and organic acid as an activator, and metallic powder. The alcohol has at least two OH groups. The organic acid reacts with the alcohol to produce ester when the metallic powder is fused. Preferably, the soldering paste includes a material for reducing a viscosity of the soldering paste.
According to a second aspect of the present invention, a soldering method for soldering an electronic component to a base member uses soldering paste including alcohol which has at least two OH groups, organic acid, and metallic powder. The electronic component is soldered to the base member by the soldering paste during a soldering reflow.
According to a third aspect of the present invention, a surface-mounted type electronic device includes an electronic component bonded to an electrode through a soldering portion, and an ester film disposed on a peripheral surface of the electrode. The ester film is a reaction product between an organic acid and an alcohol and including the organic acid therein.
The addition of the organic acid to the soldering paste allows the addition of the material to the soldering paste for reducing the viscosity of the soldering paste. As a result, the printability of the soldering paste is improved without lowering the wettability. Further, the organic acid reacts with alcohol in the soldering paste to produce an ester film using metallic powder as a catalyst during soldering reflow. The ester film incorporates therein remaining organic acid and metal salt as its reaction product. As a result, insulation reliability is improved.